var c = 0;
var t;
var timer_is_on = 0;

function timedCount() {
	document.getElementById('txt').value = c;
	c = c + 1;
	t = setTimeout("timedCount()", 1000);
}

function doTimer() {
	if (!timer_is_on) {
		timer_is_on = 1;
		timedCount();
	}
}

function stopCount() {
	clearTimeout(t);
	timer_is_on = 0;
}